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Precambrian rocks are at or near the surface in only about 10 percent of the conterminous United States, but it can reasonably be inferred that they comprise the bulk of the continental crust beneath about 90 percent of the area. They are missing or unrecognized in the accreted terranes along the Pacific margin, but form significant parts of the crust in terranes accreted to the eastern continental margin during the Paleozoic. The total area of Precambrian rocks considered in this volume is comparable to that of the exposed Precambrian of the Canadian Shield. In comparison with the lateral extent of the Precambrian craton, the thickness of the continental crust is almost insignificant. The width of the craton in the northern conterminous United States is more than half the radius of the plant; the thickness of the continental crust is less than one hundredth of the planetary radius (Fig. 1). The volume of the continental crust is less than 2 percent of the volume of the mantle beneath it. Nevertheless, Precambrian rocks contain the only available record of the assembly and evolution of the fragile continental raft that we know as North America during more than five-sixths of geologic time. Of the areas of exposed Precambrian rocks in the conterminous United States, about half have been covered by modern reconnaissance geologic mapping (scale 1:250,000 or larger); less than a quarter hve been covered by detailed modern mapping (1:62,500) or larger. Perhaps 60 percent of the concealed Precambrian has been at least sketchily explored by drilling at spacings ranging from a few kilometers to a few tens of kilometers.

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